Experiments are proposed to investigate hypothalamic control of the onset of puberty in female rhesus monkeys and guinea pigs from the viewpoint of comparative neuroendocrinology. First, a test was done to determine whether the hypothesis of "differential sensitivity to ovarian steroid" or establishment of positive feedback system "at the level of the hypothalamus" was the key factor for onset of puberty. Specifically, PMSG was administered to monkeys at age 18 months and to guinea pigs at ages 10, 25, and 35 days. Slight, but not dramatic, effects were obtained for induction of precocious puberty by PMSG treatment in both species. Furthermore, precocious development of positive feedback response to estrogen was also evaluated in ovariectomized prepubertal monkeys and in intact guinea pigs. Preliminary negative data in monkeys and positive data in guinea pigs were obtained. Secondly, secondary direct evidence of hypothalamic involvement in the onset of puberty was examined by lesion. So far, 7 monkeys at age 18 months have received lesions in the anterior or posterior part of the hypothalamus, or sham lesions. No results can be reported yet. In the guinea pig anterior hypothalamic lesions advanced precocious vaginal opening (pseudopuberty), but did not advance first ovulation. Rather, lesions delayed the onset of first ovulation and induced constant estrous anovulatory syndrome.